The FDA has approved a second drug for fibrodysplasia ossificans progressiva, an ultra rare genetic disease in which muscle, tendon and ligament gradually turn into bone. Patients effectively grow a second skeleton, one that locks joints permanently and generally cannot be removed surgically, because cutting it out tends to trigger more bone formation.
The drug is Pasatru, generic name garetosmab-grts, made by Regeneron. The FDA approved it to reduce new heterotopic ossification, the term for bone forming outside the skeleton, and to reduce clinician-assessed disease flare-ups in adults. It carried Breakthrough Therapy, Fast Track, Orphan Drug and Priority Review designations.
The price arrived with it. Regeneron told Managed Healthcare Executive the annual list price is about $1.4 million for a patient of the average weight in the clinical trial at the higher dose, with a launch range from $693,000 to $2.1 milliondepending on dose and patient weight. Roughly 900 people worldwide are diagnosed with the disease.
Bone That Grows Where It Should Not
FOP is caused by a mutation in a receptor called activin A receptor type 1, which normally helps control new bone growth. When the receptor misbehaves, connective tissue turns to bone in places it never should, producing limited movement, deformity, severe disability and early death.
The disease follows a brutal timeline. Most patients require a wheelchair by age 30, and the median age of survival is 56. Minor injuries, falls and even intramuscular injections can set off flare-ups that seed new bone.
Pasatru is a fully human monoclonal antibody that blocks activation of the faulty receptor by targeting activin A, a protein Regeneron researchers identified as central to how these bone lesions form. It is given by intravenous infusion over 60 minutes once every four weeks. The recommended starting dose is 10 mg per kilogram of body weight, which can be lowered to 3 mg per kilogram if the higher dose is not tolerated.
Lesion Counts Fell Sharply in a 63 Person Trial
The approval rests on OPTIMA, a randomized, double-blind, placebo-controlled phase 3 trial in 63 adults. Participants received either dose of Pasatru or placebo by infusion every four weeks for 56 weeks. Sixty-one continued into an extended follow-up phase on the same assignment.
The main measure was the number of new heterotopic ossification lesions detected by full body low dose CT scans at week 56. The separation was large. Among 23 patients on the 10 mg per kilogram dose, 2 new lesions formed. Among 19 patients on the 3 mg per kilogram dose, 1 new lesion formed. Among 21 patients on placebo, 19 new lesions formed. Those figures amount to reductions of 90 percent or more against placebo.
Kathryn Dahir, a professor in the division of endocrinology, diabetes and metabolism at Vanderbilt University and a primary OPTIMA investigator, said in a company statement that for people living with FOP, "every irregular new bone formation is a step toward disability."
Sixty-three participants is small by ordinary drug trial standards. For a disease affecting a few hundred people worldwide, it represents a substantial share of the eligible population.
Clinician-Rated Flares Improved While Patient-Reported Flares Did Not
The flare-up results are more complicated than the headline suggests, and the detail matters for anyone weighing this treatment.
Clinician-assessed flare-ups over 56 weeks numbered 9 in the higher dose group, 53 in the lower dose group and 66 with placebo. The 10 mg per kilogram dose cut clinician-rated flares dramatically, while the 3 mg per kilogram dose barely moved them despite performing well on the lesion count.
More significant is what patients themselves reported. Changes in the proportion of patients reporting their own flare-ups through week 56 were not significantly different between Pasatru and placebo. A drug can therefore reduce what a clinician records and what a scan detects without patients noticing a corresponding change in their experience over that window. Whether preventing lesions translates into preserved function over years is a question this trial length cannot answer.
Safety carries real weight here too. Pasatru has a warning for fetal harm during pregnancy, and patients who can become pregnant are advised to use effective contraception during treatment and for six months after the last dose. It also carries warnings for skin and soft tissue infections requiring treatment or hospitalization, and for nosebleeds requiring medical intervention. Common adverse reactions include nosebleeds, increased hair growth, abscess and acne, with reported effects also including loss of eyebrows, mouth ulcers, folliculitis, nail infection and rash. Serious treatment-emergent adverse events occurred in two patients on the higher dose, one on the lower dose and two on placebo.
Adults Only, and a Gap in Who Gets Covered
The approval is for adults. Regeneron states it is not known whether Pasatru is safe and effective in children, which leaves a meaningful hole given that FOP begins in early childhood and much of the damage accumulates young.
The first approved FOP drug fills part of that gap and creates another. Ipsen's Sohonos, generic name palovarotene, was approved in 2023 to reduce new abnormal bone formation and is indicated for girls 8 and older and boys 10 and older. It launched at $624,000 per year based on a 5 mg daily dose. Between the two drugs, children below those age thresholds still have no approved option, and the two products work through entirely different mechanisms. Several other candidates remain in mid-stage testing.
For the small number of American families affected, the practical questions are insurance and infusion access. Pasatru can be given across a range of care settings including home infusion where appropriate, which matters for patients whose mobility is already limited. Regeneron says its myRARE program covers benefit verification and information about potential financial assistance. A company spokesperson said Regeneron weighed what the approval means for the FOP community alongside decades of research investment when setting the list price.
List price is not what most patients pay, and coverage decisions for ultra rare disease drugs are typically made case by case rather than through standard formulary rules. Patients and caregivers facing a denial can ask the prescribing specialist about prior authorization requirements, the appeals process and manufacturer assistance. Nobody should start, stop or change treatment based on a news report.
Key Questions Answered
What is fibrodysplasia ossificans progressiva?
An ultra rare genetic disease caused by a mutation in activin A receptor type 1. Connective tissues including muscle, tendon and ligament gradually turn into bone, causing limited movement, deformity, severe disability and early death. About 900 people worldwide are diagnosed with it.
What did the FDA approve?
Pasatru, generic name garetosmab-grts, from Regeneron. It is approved to reduce new bone formation outside the skeleton and to reduce clinician-assessed flare-ups in adults with FOP. It is given by intravenous infusion once every four weeks.
How well did it work?
In a 63 person trial, 2 new lesions formed among 23 patients on the higher dose and 1 among 19 patients on the lower dose, compared with 19 new lesions among 21 patients on placebo. Clinician-assessed flare-ups were 9, 53 and 66 respectively.
Did patients feel a difference?
Not measurably within the trial. Changes in the proportion of patients reporting their own flare-ups through 56 weeks were not significantly different between Pasatru and placebo, even though clinician-assessed flares and scan-detected lesions improved.
What are the main safety concerns?
Warnings cover fetal harm during pregnancy, skin and soft tissue infections that may require hospitalization, and nosebleeds requiring medical intervention. Common reactions include nosebleeds, increased hair growth, abscess and acne.
What does it cost?
Regeneron reports an annual list price of about $1.4 million based on the average trial patient weight at the higher dose, with a launch range of $693,000 to $2.1 million depending on dose and weight. List price generally differs from what an insured patient pays.
Is there anything available for children?
Not from this approval, which covers adults only. Sohonos, approved in 2023, is indicated for girls 8 and older and boys 10 and older. Children below those ages currently have no approved treatment option.
